1. Field of the Invention
This invention relates to resinous compositions, and more particularly, to an anti-static resinous composition which has a permanent anti-static property for preventing the resultant plastics from accumulating electrostatic charges thereon. Still more specifically, the anti-static resinous composition of the invention comprises a hydrophilic copolymer which has good heat-stability when mixed with thermoplastic under high temperature process conditions. Further, the hydrophilic copolymer can be compatibly mixed with the thermoplastic without the need to add any compatible agents thereto. The anti-static resinous composition is formed by mixing the hydrophilic copolymer with the thermoplastic at various ratios. The plastic articles formed from the anti-static resinous composition of the invention possesses excellent permanent anti-static property and will not suffer from disintegration or blooming.
2. Description of Related Art
Thermoplastics are widely utilized in the industry for making various kinds of plastics product with excellent insulation and medical properties such as high modulus, high impact resistance etc. They can be made by various processes such as extruding, rolling, injection and heat-formation.
One drawback to the thermoplastics, however, is that, due to their insulating quality, static electricity easily accumulates on the plastic articles made therefrom, which can cause electro-static damage to the related circuit parts in, for example, copy machines, faxes, printers, to name just a few, that are enclosed in plastic casings. Moreover, dust can be easily attracted by the static electricity and accumulate on the plastic casings or covers of these office machines. In printing factories, for instances, the static electricity on the plastic parts of a high-speed printing machines can cause the paper sheets passing therethrough at high speeds to be attracted astray, usually causing the so-called paper path jam in the machine. In a winding machine for tapes made of plastics, for instances, the static electricity produced due to the high-speed winding of the tapes, when accumulated to a certain great amount, can cause a sudden electric-static discharge in the form of an electric arc, which could then cause electrical shock to the operating personnel or damage to the machines.
In the semiconductor industry, the transistor elements on IC chips, due to their extremely small sizes, are very sensitive to the static electricity accumulated on the plastic packages. A static charge with a potential in the range of 50 to 300 V (volt) can cause severe damage to the transistor elements in the ICs. A human body carries an average of 1 to 2 kV (kilovolt) of static electricity. When touching the ICs by hand, the static electricity on the human body can cause damage to the transistor elements therein. Therefore, during the manufacture and transportation of the ICs, they should be put in containers made of anti-static materials.
Several solutions have been proposed to prevent the electrostatic damage. The first solution is to coat a layer of anti-static material on the surfaces of the plastics by, for example, a dipping, wiping, or spraying process. The anti-static material is usually a moisture-absorbing interfacial agent which can prevent the accumulation of static electricity on the coated plastics. One drawback to this kind of anti-static material is that the anti-static effect thereof decreases as the ambient humidity decreases. Moreover, the coating of this anti-static material can easily peel off the surfaces of the plastics. For a circuit enclosed by a plastic casing, this could result in the undesired consequence of contaminating the circuit elements enclosed therein. Should the coating of the anti-static material peel off, the plastic casing must be surface treated again so as to form a new coating of the anti-static material thereon.
The second solution for enhanced anti-static effect is to mix the thermoplastic with a low molecular weight anti-static agent in the process of making the plastics. The molecules of this anti-static agent can migrate to the surface of the plastics and attract the moisture molecules from the ambient atmosphere to form a thin water film on the surface of the plastics. This thin water film can prevent the buildup of static electricity on the surface of the plastics. Various compounds have been proposed to serve as this kind of anti-static agent, as the N-oxyethylated alkylamines disclosed in U.S. Pat. No. 3,575,903; the alkanolamines and polyalkylene glycols disclosed in U.S. Pat. No. 3,525,915; and the lauric diethanolamide disclosed in U.S. Pat. No. 3,873,645. One major drawback to these anti-static agents, however, is that, the ingredients thereof could filtrate out to the surface of the finished plastic articles, thus causing the undesired effect of blooming on the same. Moreover, after a long period of use, the anti-static effect may decrease.
The third solution for enhanced anti-static effect is to add a high molecular weight anti-static agent in the process of making the plastics. This kind of anti-static agent can allow the plastics to have a permanent anti-static effect and is classified in terms of its compatibility with the thermoplastic into two types. The first type has less compatibility with the thermoplastic while the second type has more compatibility.
The first type of the anti-static agent, when mixed with the thermoplastic, enrich a highly dense continuous phase on the surface of the plastics, allowing the anti-static agent to exhibit enhanced anti-static effect on the same. Various compounds have been proposed to serve as this kind of anti-static agent, as the polyether ester amide disclosed in U.S. Pat. No. 5,338,795 and Japan Patent Heisei-4,337,344; and the epichlorohydrincopolyer disclosed in U.S. Pat. Nos. 4,588,733, No. 4,775,716, No. 4,857,590, and No. 5,216,046. To achieve the highly dense continuous phase on the surface of the plastics, however, a suitable compatible agent must be added to the mixture of the anti-static agent and the thermoplastic. Without the adding of the compatible agent, the thermoplastic will lose its characteristics.
The second type of the foregoing anti-static agent is formed by copolymerization of a vinyl monomer containing polyethylene oxide chains with an ordinary vinyl monomer. When mixed with thermoplastic, the vinyl monomer ingredient in the anti-static agent provides compatibility, while the polyethylene oxide ingredient provides the desired anti-static effect. Since this type of anti-static agent has more compatibility with the thermoplastic, it can exhibit a continuous phase on the surfaces of the plastics, allowing the anti-static agent to provide a permanent anti-static effect and excellent mechanical properties. The copolymerization is customarily an emulsion polymerization process, which allows the anti-static agent to possess the qualities of high impact-resistant capability and matting. Various compounds have been proposed to serve as this kind of anti-static agent, as U.S. Pat. No. 4,543,390; Japan Patents Showa-60 144,314; Showa-60 195,143; Showa-61 261,344; Showa-62 48,756; and Heisei-1 126,358.
From the foregoing, it is apparent without question that the permanent anti-static agent will be a key player in the future of plastic applications. However, although the high molecular weight copolymers such as polyether ester amide, epichlorohydrin copolymer, and high-molecule-weight polyethylene oxide (high M.W. PEO), are effective permanent anti-static agents, they exhibit some drawbacks when mixed with the thermoplastic. For instance, compatible agents should be added to some of the anti-static agents to preserve the mechanical properties of the resultant plastics. Some of the anti-static agents require the addition of alkaline salts, interfacial agents, or other additives to help them exhibit the anti-static property. When interfacial agents are added, the processability of the resultant plastics will be limited. The anti-static agents made by emulsion polymerization can be provided with backbones that are compatible with the thermoplastic and provide side chains that have anti-static property, so that, when they are mixed with the thermoplastic, no compatible agents are required while nonetheless providing excellent anti-static properties. One drawback to these anti-static agents, however, is that, since they are made by emulsion polymerization, the emulsifying agent will be left as a residue therein during separation, purification, or mixing processes. The residue of the emulsifying agent will cause the resultant plastics to have poor appearance. In addition, the residue on the surface of the resultant plastics causes the resultant plastics to have poor post-processability.